The Corangamite Water Skink was first discovered in 1963, near Lismore in Victoria. Since then it has been found at several locations on the Victorian Volcanic Plain between Colac and Lake Bolac. Its historical distribution is not known, but it is highly likely that the taxon once occurred at other locations on the basalt plains. The main factors involved in the decline of E. t. marnieae appear to be habitat loss and fragmentation, and degradation of remaining areas of habitat by a range of processes including changed grazing regimes, weed invasion, rock removal, changed hydrology and/or water quality, cropping, use of agricultural chemicals and the impacts of introduced animals (foxes, cats, rabbits, mice, sheep, cattle) either by predation or by grazing (Robertson and Peterson 2000). Native grassland on the Victorian Volcanic Plain is one of the most threatened ecological communities in Australia, with less than 1% of its original extent remaining (Lunt et. al. 1998), and this community has been listed as Critically Endangered under the EPBC Act (DEWHA 2009).
The taxon is extinct from one site each at Lake Bolac and Lismore, and has undergone declines in other areas, generally coinciding with the prolonged drying of adjacent wetlands (Scroggie 2005; Peterson unpublished data). The population at the first site discovered (near Lismore) was probably destroyed during the rock clearing activities which produced the original specimens (Hutchinson and Rawlinson 1995). Remaining sites are patchily distributed and isolated, with the bulk of sites having very small areas of available habitat. The majority of populations are comprised of two or more sites or subpopulations that historically were probably connected by continuous habitat. Dispersal between fragmented sites now seems unlikely. Fragmentation of most sites has occurred as a result of processes such as vegetation clearing, rock removal and the drainage of wetlands. For small isolated populations, stochastic ecological and/or genetic isolation effects could become important, as could the potential for inbreeding depression. Synergistic effects of combinations of these threats may also be important. For example, cat predation may be more important in small populations close to urban areas.
Most populations of E. t. marnieae are found on private land generally used for agriculture, including dairy farming, grain cropping and beef and sheep farming. Aspects of all of these agricultural practices threaten the associated E. t. marnieae populations. These threats continue, to varying degrees, at all known sites. Populations are now extremely fragmented and many are very small. The native shrubs and forbs in these rocky grassland/wetland habitats may be particularly susceptible to grazing. As these shrubs and some forbs have been shown to be important to E. t. marnieae (Peterson 1997; 2000), it is likely that heavy grazing will be detrimental to E. t. marnieae. Grazing may also be detrimental to neonatal and juvenile skinks as the microhabitat they tend to inhabit is heavily vegetated and consequently heavily grazed. Appropriate grazing regimes have yet to be determined, however, it has been seen as prudent to exclude grazing from E. t. marnieae habitat wherever possible (Peterson 2006).
Irrigation for agricultural purposes through ground water extraction, in conjunction with the extended drought, has caused the drying of a number of water bodies associated with E. t. marnieae. In a number of cases, this has coincided with a decline in the adjacent E. t. marnieae populations (Peterson 2000, 2001). The loss of the cool moist refuges and overall deterioration in wetland habitat quality is thought to have driven these declines. Continued ground water extraction, in particular in concert with drought and/or climate change poses a significant threat to E. t. marnieae.
The use of agricultural insecticides associated with cropping may be detrimental to E. t. marnieae by impacting invertebrate prey availability at sites adjacent to crops. South-western Victoria has undergone a significant change in land use from broad acre grazing to cropping over the past decade, heightening this threat to E. t. marnieae.
The removal of basalt rocks to increase grazing land, enable pasture improvement and/or to provide rocks for the landscape garden market threatens E. t. marnieae by either directly removing habitat or by reducing dispersal routes and isolating populations. Populations have already been fragmented at most sites, however further rock removal threatens sites where continuous habitat still persists. The loss of small wetlands and moist depressions and the resulting increase in inter-wetland distance may also further isolate populations (Gibbs 1993).
Long term, the impacts of climate change have the potential to be catastrophic to E. t. marnieae. Even in the absence of an ever increasing agricultural demand on water resources, the prolonged drying of wetlands predicted to be associated with climate change will potentially impact significantly on E. t. marnieae. Due to the hydrophilic nature of the taxon and its reliance on wetlands and associated moist microclimates, the impacts of climate change coupled with the agricultural demand for water resources, have the potential to be catastrophic to the subspecies and associated wetland ecosystems.
As many populations of E. t. marnieae are now severely fragmented, it is unknown whether the lizard will persist in the long-term in reserved or managed areas, or whether active intervention will be required. This will depend upon the size of the reserves or managed areas, on the threats present, and on the management regimes implemented. The size of most extant populations may not be large enough currently to prevent major restrictions of genetic variability, especially in the absence of natural gene flow, and management must be cognisant of this. Molecular research is at present being conducted to examine small scale movement and dispersal (J. Sumner pers. comm. 2009), the results of which will be used for developing various management requirements.
The life history of E. t. marnieae appears such that relatively quick population growth may be possible given adequate areas of habitat and amelioration of threats. Rapid increases in E. t. marnieae numbers were observed at three sites over a short period (i.e. three years) following the exposure of new habitat (Peterson 2001). Animals have also inhabited newly created habitat shortly after works were completed (G. Peterson unpubl. data 2004).